1). Field of the Invention
The invention relates to crosslinkable dispersion powder compositions which are redispersible in water, to processes for their preparation and to their use.
2). Background Art
Dispersion powder compositions which are based on homo- or copolymers of ethylenically unsaturated monomers and are redispersible in water are known. Such a dispersion powder composition is prepared by spray drying the corresponding aqueous plastics dispersions in a stream of hot air. The dispersion powders are suitable as additives for hydraulic binders in the construction materials industry, and furthermore such products are employed as binders in coating compositions or adhesives.
Crosslinkable dispersion powders are known from EP-A 149098 (U.S. Pat. No. 4,859,751). The polymers mentioned therein contain N-methylolacrylamide or N-methylolmeth-acrylamide units as crosslinking functions. The disadvantages of such polymers are on the one hand that formaldehyde is liberated when N-methylolamide units are crosslinked, but above all N-methylolamide-containing polymers crosslink only in acids, if appropriate by means of addition of crosslinking catalysts. However, the main field of use for crosslinkable dispersion powders which are redispersible in water lies in the building sector; here, it must also be possible for crosslinking of the binder to take place, above all, in the alkaline range.
There was therefore the object of providing crosslinkable dispersion powders which are redispersible in water and also crosslink in the basic range, but without already crosslinking completely during preparation of the dispersion powder, that is to say during the polymerization or drying.
Known vinyl ester copolymers which can also be crosslinked in the basic range are those which contain vinylalkoxysilane units. DE-A 3727181 (U.S. Pat. No. 4,959,249) describes aqueous dispersions of polymers which comprise vinyltrialkoxy- or alkylvinyldialkoxysilanes as crosslinking units. Aqueous copolymer dispersions analogous to these which are based on alkyl acrylate polymers with acryloxyalkyl (trialkoxy) silane units are known from U.S. Pat. No. 3,706,697. The fact that in copolymer dispersions with alkoxysilane units as crosslinking functions these are readily hydrolysed and crosslinked by condensation even at relatively low temperatures of about 50.degree. C. presents problems here.
To prevent premature crosslinking of aqueous dispersions of alkoxysilane-substituted copolymers, EP-A 485057 proposes using aqueous dispersions which, in addition to water-insoluble, alkoxysilane-substituted copolymers, also comprise polar, low molecular weight alkoxysilane copolymers. To improve the storage stability of aqueous dispersions of polymers with alkoxysilane functional units, U.S. Pat. No. 5,214,095 proposes polymerizing these in the presence of condensable siloxane precursors, polysiloxanes which can envelop the alkoxysilane units like protective groups being obtained.
EP-A 493168 relates to dispersion powders obtained by spray-drying a mixture comprising aqueous polymer dispersion and liquid organopolysiloxane.
CH-A 499650 describes a process for preparing polyvinyl ester-based dispersion powders where silica powder is added before spray-drying to the dispersion yet to be dried, in order to prevent caking of the polymer particles in the course of spray-drying.
EP-A 601518 relates to redispersible polymer powders which are based on (meth)acrylates, comprise from 1 to 15% of ethylenically unsaturated comonomer units and are dried in the presence of polyvinyl alcohol as atomizing aid.
Surprisingly, the abovementioned object has been achieved with dispersion powders of copolymers with alkoxysilane functional units, although against the background just discussed, during the required drying of the copolymer dispersions in a temperature range from 55 to 100.degree. C., complete crosslinking of the copolymers had to be expected.